New York - Two hours after Fernando Tinoco started his new job at a meatpacking plant in Chicago, he was escorted by security guards to the office and fired.

The reason: Company officials had entered his Social Security number into the Department of Homeland Security's E-Verify system. It's a mostly voluntary program designed to give employers a fast, easy way to check a person's immigration status. Mr. Tinoco's information came back as a "tentative non-confirmation," meaning that he may not be a citizen. He was shown the door.

But Tinoco is a citizen and has been since 1989. Immediately after his firing a few months ago, he went to a Social Security office and got a letter confirming his legal status. It was too late.

"I went back and the security guard chased me away, told me not to come back to the company because I was fired," he says in a phone interview.

President Bush's recent executive order mandating that all federal contractors use E-Verify and legislation pending in Congress that would make the program mandatory for all employers nationwide have heightened concerns among critics that thousands of legal Americans will be unfairly denied jobs. That's because E-Verify relies mainly on the Social Security database, which the Government Accountability Office has found to be fraught with errors. Studies have also shown that almost half of employers who are already using E-Verify are not abiding by rules designed to protect citizens like Tinoco.

Advocates acknowledge that E-Verify's 94 percent accuracy rate could be improved, but they insist that its benefits outweigh any imperfections. They contend that it's an easy, straightforward way for employers to comply with immigration law. Better education of employers can ensure it's used properly, they say.

In the middle are many immigration experts and economists. Worksite enforcement, they say, is crucial to controlling illegal immigration. But they also note that America's current immigration system is broken and not meeting the needs of the economy. That's why there's a steady flow of illegal, low-wage workers entering the US. These experts are concerned that imposing E-Verify nationwide now without broad immigration reform would severely damage the economy.

"We have significant sectors of the economy that need large numbers of low-­skill workers, yet we don't have legal channels for these immigrants to come in," says Judy Gans, manager of immigration policy at the Udall Center for Studies in Public Policy at the University of Arizona in Tucson. "The question is: What are we doing to the economy by imposing worksite enforcement before the legal channels are there to meet the economy's needs?"

E-Verify used to be called the Basic Pilot/Employment Eligibility Verification program. It was created by Congress in 1997 as a voluntary pilot program to give employers an electronic way to verify employees' Social Security numbers. It's now operated by the Department of Homeland Security (DHS) in partnership with the Social Security Administration and is used by almost 70,000 employers nationwide. It's currently voluntary, except in a handful of states like Arizona.

Other states are wary of E-Verify. Illinois even has a law forbidding employers from using it because of concerns about its accuracy, although the state has agreed not to enforce its law until a court case challenging it is resolved. California legislators are considering a similar ban on the use of the program.

E-Verify advocates say the program has enough safeguards to protect citizens. Workers who are given a "tentative non-confirmation" – meaning there's a problem with their Social Security number – have eight working days to clear up discrepancies in the government's database, they note. And employers who use E-Verify are held harmless it if turns out they unknowingly hired an illegal immigrant.

"If you're an employer, you're no longer required to be a document expert," says Bob Dane, a spokesman for the Federation for American Immigration Reform in Washington. "With E-Verify, you can tap into an automated Internet database that runs against almost 500 million records. It's fast, easy, and free to use. What's not to like?"

Critics have found quite a bit not to like. They note that a study commissioned by DHS itself found that for every thousand names put into the system, 58 come back as tentative nonconfirmation. Of those, about five people successfully contest the finding that they're not legally eligible to work in the US. DHS officials and advocates like Mr. Dane believe that means the other 53 applicants are probably illegal and the system is discouraging them as it should. But critics note that there's no way to know if those 53 were in the US illegally. The study commissioned by DHS also found that a substantial number of employers did not follow the E-Verify rules designed to protect citizens.

"We really don't know what the situation is with that 5.3 percent [who don't contest their tentative nonconfirmations]," says Jim Harper, director of information policy studies at the Cato Institute in Washington.

He did an analysis using the same Social Security database that E-Verify uses. The government acknowledges that the database has an error rate of 4.1. That means that about 17 million people's names may not be exactly correct or there was an error when the information, like date of birth, was entered. Though they are here legally, those residents would come back as "tentative non-confirmations."

"As a matter of simple math, that means that if E-Verify were to go national, on the first day 1 in 25 legal new hires would be bounced out of the system and asked to go down to the Social Security office and straighten out the problem," he says.

That raises Mr. Harper's broader concerns about the program – that it would encourage employers and workers to operate "under the table" and that it could prompt even more identity theft and document fraud. "Conservatives are supposed to want people to work – not on welfare, not working under the table," he says. "Here's a system where we really don't know what's happening with 5.3 percent, but it looks like more than 1 in 100 lawful employees are being sent packing."

But supporters argue that clearing up discrepancies in the Social Security database is a "public service." They say there could be some initial disruptions to the economy, including a substantial loss of tax revenue from illegal workers who are now paying taxes. But they argue there will be long-term benefits to the economy.

"Of course some portion of illegals working on the books will stop doing so and either start working under the table or go home," says Mark Krikorian, executive director of the Center for Immigration Studies in Washington, D.C. "Either one is going to result in their no longer paying taxes. But enforcing the law is supposed to get those people out of their jobs: They're not here legally."